Fiber-optic probes make it possible to collect optical information such as Raman spectra without having to place the material being characterized inside a spectrometer housing. Such probes therefore simplify the interfacing of spectroscopic systems to chemical processes, and allow analytical instruments to be located remotely from environments in need of spectroscopic monitoring.
The first remote fiber optic probes for Raman spectroscopy were reported by the McCreery group in the early 1980's. Their design used a single optical fiber to deliver laser light to the sample and a single optical fiber to collect light scattered by the sample. More specifically, divergent laser light from the laser delivery fiber was used to illuminate the sample, and light scattered from the sample within the acceptance cone of the collection fiber was transmitted back to the spectrograph. The efficiency of exciting and collecting Raman photons from any individual point in the sample was poor, but the integrated Raman intensity over the unusually large analysis volume compared favorably with the more traditional imaged illumination and collection configurations.
McCreery's dual-fiber Raman probe offered important benefits for remote and routine spectroscopy: 1) the sample could be distant from the Raman instrument, 2) no sample alignment was necessary once the probe was aligned to the spectrograph, 3) the probe could be less 1 mm in diameter, making Raman measurements possible for samples with limited accessibility, 4) the probe could be placed directly in hostile samples (corrosive, hot, etc.) since only silica and the encapsulation material were exposed, and 5) multiple measurements could be made simultaneously by placing multiple collection fibers along the slit height of the spectrograph.
Several improvements to the McCreery Raman probe have more recently been reported. Instead of using just one collection fiber, multiple fibers have been used to increase the collection efficiency, as shown in FIG. 1A. For example, 6 fibers, each having the same diameter as the excitation fiber, may be grouped around the excitation fiber to form a single circular layer, as shown in U.S. Pat. No. 4,573,761. Eighteen fibers, each having the same diameter as the excitation fiber, may also be grouped around the excitation fiber as two circular layers, and so on, though successive layers tend to be less effective at collecting Raman photons than the first layer.
The performance of the McCreery type probe can also be modified for improved collection efficiency and/or working distance by changing the overlap between the emission cone of the excitation fiber and the collection cones of the collection fibers. An early realization of this idea, as disclosed in U.S. Pat. No. 4,573,761, angled the collection fibers such that their optic axes intersected the optic axis of the illumination fiber, as shown in FIG. 1B. This increased the overlap of the excitation and collection cones close to the tip of the fiber probe, where the excitation and collection of Raman photons was most efficient.
The same concept was later implemented in a different way by O'Rourke and Livingston, who ground the tip of the probe into a cone shape, as discussed in U.S. Pat. No. 5,402,508, and illustrated in FIG. 1C. This shape was equivalent to putting prisms (or more correctly, axicon sections) on the collection fibers so that the optic axes of the collection cones crossed the optic axis of the excitation fiber.
One further variation of the McCreery probe design is to use collection fibers having a different diameter than the excitation fiber. This additional variable is useful for changing the working distance of the probe and the fiber coupling to the spectrograph.
One disadvantage of the various arrangements described thus far is that the sample must be very close to the probe tip to realize any significant collection efficiency.